It is already known from the technical literature that anodic oxidation may be carried out to produce an oxide coating which protects against corrosion and wear on the metal surface of certain metal alloys. This process has great technical significance for the treatment of metal surfaces which may be coated in this way with an oxide coating which protects against corrosion and wear (for more about this, compare Kraftfahrtechnisches Taschenbuch, Bosch, 21st Edition). Anodic oxidation is utilized, above all, in the surface treatment of an aluminum workpiece to reinforce the natural oxide film and thus to achieve a so-called anodized coating.
An aluminum part is wired as an anode and, together with a counter-electrode, receives a constant or pulsating direct current in an electrolyte. The transformation (oxidation) of metallic aluminum to aluminum oxide forms a coating on the surface, which protects against corrosion and wear. The structure and thickness of the coating are essentially a function of time, temperature and current density.
As a rule, the aluminum oxide coating is very hard and brittle, and therefore it cannot be prevented from chipping, for a corresponding exterior influence of force on the anodized coating, which is undesirable in certain applications.
It is also known that the anodizing of special locations of a component may be prevented or limited to a minimum by means of masking with an insulating lacquer or with protective screens.
However, these methods are very costly for industrial scale manufacture and cannot be automated for the application described, or may be automated only at great cost.